1. Field of the Invention
This invention relates to an improved process combination for the conversion of hydrocarbons, and more specifically for an improved reforming/aromatization process.
2. General Background
The widespread removal of lead antiknock additive from gasoline, the rising fuel-quality demands of high-performance internal-combustion engines, and growing demands for chemical aromatics have compelled petroleum refiners to install new and modified processes to increase the severity of processing gasoline-range feedstocks. Refiners have relied on a variety of options to upgrade the gasoline pool, including higher-severity catalytic reforming, higher FCC (fluid catalytic cracking) gasoline octane, isomerization of light naphtha and the use of oxygenated compounds.
Catalytic reforming, or aromatization as the modern selective version often is termed, is a major focus since this process generally supplies 30-40% or more of the gasoline pool as well as most of the chemical benzene, toluene and xylenes. Increased aromatization severity often is accompanied by a reduction in pressure in order to obtain high yields of aromatics and gasoline product from the process. Both higher severity and lower pressure promote the formation of olefins in aromatization, and the 1-2+% of olefins in modern reformates contribute to undesirable gum and high endpoint in gasoline product as well as high clay consumption in aromatics-recovery operations.
Aromatization product often is clay treated to polymerize the small concentrations of olefin present [see, e.g., U.S. Pat. No. 3,835,037 (Fairweather et al.)]. This procedure forms heavy polymer, undesirable in gasoline component since it forms deposits in engines; further, the clay is costly and disposal of spent clay may be difficult and expensive. A problem facing workers in the art, therefore, is to discover a method of olefin removal which does not suffer the above drawbacks.
Considering selective hydrogenation of olefins, U.S. Pat. No. 3,869,377 (Eisenlohr et al.) teaches elimination of aliphatic unsaturates from a reformate by cooling a reaction mixture from hydroforming which contains hydrogen and aromatics and passing this mixture in gaseous state through a reactor containing a catalyst comprising oxides of Group 6 and/or 8 metals [preferably cobalt and molybdenum]. Russian disclosure SU1513014-A (Maryshev et al.) teaches hydrogenation of reforming products at elevated temperature in the presence of aluminum-platinum catalysts. Hydrogenation of olefins by adding a reactor within the hydrogen circuit of an associated unit suffers the disadvantage of adding pressure drop to the circuit, and also does not provide control of the ratio of hydrogen to olefin in the saturation zone, and does not reduce the concentration of hydrogen in separator liquid to a subsequent fractionator as in the present invention.
Selective hydrogenation of small quantities of alkenes in xylene-isomerization product, using a hydrogenation metal supported on a crystalline borosilicate molecular sieve, is disclosed in U.S. Pat. No. 5,015,794 (Reichmann). U.S. Pat. No. 4,885,420 (Martindale) teaches hydrogenation of relatively large concentrations of olefins in a light (C.sub.2 -C.sub.5) hydrocarbon stream, wherein the concern in the present application relating to aromatics saturation is not an issue.